The process of machining called Electrical Discharge Machining (EDM) is used for cutting holes, slots, and other irregular shapes which are difficult to machine with conventional cutting tools. The workpiece must be electrically conductive and the material is removed in a controlled manner by utilizing highfrequency electrical sparks to melt or vaporize the material being removed. The spark discharge results from a pulsing of direct current between the positively charged workpiece and the negatively charged tool which is generally a graphite form. A gap of 0.0005 to 0.020 inches is used with the parts surrounded by a dielectric fluid bath. Each spark melts or vaporizes a small portion, the workpiece producing cratering which erodes the portions of the workpiece to be removed.
The graphite used for the negative electrode has a rather short tool life and requires frequent replacement. The graphite tool forms are shaped by a steel cutting tool which has generally the shape of the part to be ultimately formed. The forming of this cutting tool by present methods is essentially a 90-hour process involving a plurality of steps as follows:
1. Forming a master die by hand machining to print specifications;
2. Making a plastic master die from the steel master die in a pouring process;
3. Using the plastic master as a form on a tracer machine to cut out graphite blocks identical to the plastic master;
4. Using one or more of the graphite blocks in an EDM machine to form a steel tool from a solid block, the cutting surface having a rough texture as a result of the EDM process;
5. Using the steel tool to form graphite electrodes by orbital abrading machining, that is, abrading away the graphite by contact with the rough surface on the cavity walls of the steel tool; and
6. Finally using the graphite electrodes in an EDM process to form the finished steel products.
A major drawback of the above method is that the steel cutting tool, which has a rough cutting surface, loses its sharpness after 25 to 50 electrodes are cut and requires a much longer cutting time which becomes too costly. Resharpening the tool in the numerous steps above described takes time and results in a tool which has a larger cavity as a result of eroding to get the cutting texture required. Thus, accuracy in the final product becomes more difficult. After about two resharpenings, the steel tool has to be scrapped and another made using the time consuming steps above recited.
The present invention contemplates a method of resharpening or originally creating a graphite electrode cutting tool which requires about one hour as compared with the standard procedures. Thus, time is saved and materials are saved and the resulting tools are equally satisfactory.